Two-dimensional Ferroelectric Alpha-In2Se3 Exhibits Robust Valence Band Inversion at Gamma Point

Two-dimensional ferroelectric materials are gaining prominence due to their diverse electronic properties and potential for advanced technologies, and alpha-In2Se3 stands out as a particularly promising candidate. This material exhibits both in- and out-of-plane ferroelectricity alongside a high photo-response, making it attractive for a range of device applications. Now, Geoffroy Kremer (Institut Jean Lamour, CNRS-Université de Lorraine, France), Aymen Mahmoudi (Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, France), Meryem Bouaziz (Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, France) and their colleagues have performed a detailed investigation of the electronic structure of rhombohedral alpha-In2Se3. Combining angle-resolved photoemission spectroscopy with theoretical calculations, the team demonstrates a unique ‘bow-shaped’ dispersion of the valence band, a result of a robust inversion of its parabolicity, and reveals an indirect band gap of approximately 1. 25 eV. These findings, coupled with evidence of significant electron doping at the surface, provide a deeper understanding of the material’s electronic properties and pave the way for its exploitation in future electronic and photonic devices.

Layered Perovskites as Novel Ferroelectrics

Two-dimensional ferroelectric materials offer a diverse range of electronic properties influenced by their chemical composition, layer count, and stacking arrangement. These materials hold considerable promise for next-generation electronic devices, potentially enabling functionalities beyond those achievable with conventional semiconductors. Recent theoretical work suggests that layered perovskite oxides, particularly those with distorted atomic arrangements, may exhibit strong ferroelectric responses in 2D form. These materials offer the potential for high polarization and tunable properties through careful control of their composition and layer structure. This work focuses on the synthesis and characterization of ultrathin films of bismuth-layered perovskite oxides, aiming to establish a clear understanding of how structural distortions, electronic structure, and ferroelectric behaviour are interconnected. By controlling film thickness and composition, the team seeks to optimize ferroelectric properties and demonstrate the potential of these materials for advanced electronic devices, contributing to the field of 2D ferroelectronics and paving the way for novel functional materials.

Rhombohedral In2Se3 Electronic Structure via ARPES and DFT

Researchers meticulously investigated the electronic structure of rhombohedral alpha-In2Se3, a two-dimensional material exhibiting promising ferroelectric properties, by combining angle-resolved photoemission spectroscopy with theoretical density functional calculations. Experiments began with high-quality alpha-In2Se3 single crystals, carefully cleaved under ultra-high vacuum conditions. Angle-resolved photoemission spectroscopy measurements were performed at 80K, mapping the energy and momentum of emitted electrons to reveal details about its band structure. Simultaneously, theoretical calculations were conducted using the Quantum ESPRESSO software package, incorporating spin-orbit interaction and Van der Waals interactions to accurately model the material’s electronic properties.

The combined approach revealed a robust inversion of the valence band parabolicity near the center of the material’s electronic structure, manifesting as a bow-shaped dispersion with a depth of 140 ±10 meV. Furthermore, the data unveiled an indirect band gap of approximately 1. 25 eV and a high electron doping level of 5x 10 12 electrons per square centimeter, leading to surface band bending and the formation of a prominent two-dimensional electron gas. These findings provide crucial insights into the electronic properties of rhombohedral alpha-In2Se3, paving the way for the development of novel electronic and photonic technologies.

DFT Calculations of Indium Selenide Properties

This text details computational methods and supplementary information related to a materials science study focused on the properties of Indium Selenide (InSe) and related materials. The study utilizes Density Functional Theory for electronic structure calculations, employing the Perdew-Burke-Ernzerhof generalized gradient approximation and norm-conserving pseudopotentials. A k-point mesh of 16x16x1 was used for Brillouin zone integration, with a plane-wave energy cutoff of 500 eV. The Heyd-Scuseria-Ernzerhof hybrid functional was used for more accurate band gap calculations. The HSE calculations reveal a band gap of 1.

3 eV for InSe, with the valence band showing minimal dispersion and the conduction band exhibiting strong dispersion, significant for carrier mobility. The band structure is consistent across different values perpendicular to the 2D plane, as confirmed by the Density of States. Supplementary figures provide large-scale band structure calculations and energy-dispersive X-ray spectroscopy elemental maps confirming the material’s composition and homogeneity. The study also considers related 2D materials like GaSe, GaS, As 2 Te 3 , and Tellurene.

Unique Band Structure and 2DEG Formation

This research presents a detailed investigation of the electronic properties of rhombohedral alpha-In2Se3, a two-dimensional ferroelectric material with promising potential for advanced technologies.